The term “persistent memory” is used in the context of the present description and in the claims to denote any sort of semiconductor memory that both supports random-access writing and reading of data to and from the memory and retains the data when power is turned off. Examples of persistent memory that are currently in use include various types of non-volatile random-access memory (NVRAM), including flash memory, as well as battery-backed RAM devices. New types of persistent memory are in development, such as the “3D Xpoint” memory recently announced by Intel and Micron Technology.